"The thumb on the human hand has only two phalanges whereas human fingers each have 3 phalanges. This give the thumb greater movement and enables it to do things the fingers can't do. Collectively the fingers and thumbs are known as digits. The thumb is said to be opposable to the fingers.

By definition, the thumb is both a finger and not a finger. "Finger" can mean any of the four digits besides the thumb OR any digit including the thumb. It is up to the person to decide whether or not the thumb is considered a finger."